Niobium oxide/carbon (Nb₂O₅/C) composites were synthesized in a one-pot process by combining electrochemical exfoliation of graphite with anodic deposition of niobium oxide. The purpose of this study is to develop an electrolyte system that includes a precursor for oxide deposition as well as graphite exfoliation agents. We anticipated that the oxide coating on graphene morphology would provide excellent ‘wiring’ to the transition metal oxide and enable redox reactions to occur at high rates. For electrochemical exfoliation, sulfuric acid and oxalic acid have been commonly used to efficiently prepare high-quality graphene sheets in a large quantity. In the research reported here, various amounts of ammonium niobate oxalate hydrate (NH₄NbO(C₂O₄)₂-nH₂O) were dissolved in 1M sulfuric acid and oxalic acid for a systematic study. A control system of 1M hydrochloric acid, a relatively weak exfoliation agent, was also investigated for comparison.

To understand the ion species in the electrolytes, Raman spectroscopy was used to determine the relative amount of the dissolved oxalatooxoniobate. The experimental setup consists of a high-purity graphite rod as the anode and a Pt foil as the cathode. Voltages of 5 to 10V were applied across the electrodes for 30 minutes. After that, the Nb₂O₅-anchored exfoliated carbons were collected, washed, and characterized by different techniques. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to image sample morphologies and perform elemental distribution, respectively. Thermogravimetric analysis (TGA) was used to quantify Nb₂O₅ and carbon contents and X-ray Diffraction (XRD) was used to identify Nb₂O₅phases. Raman spectroscopy was also used to determine carbon-carbon bonding quality while transmission electron microscopy (TEM) was used to image the oxide nanoparticle distribution on the carbon nanostructures.

The complex NbO(C₂O₄)₂H₂O⁻ in sulfuric acid was successfully converted into Nb₂O₅ on the exfoliated graphene sheets. In contrast, NbO(C₂O₄)₃^3- in oxalic acid prevented graphite from exfoliating by blocking the pathways for  intercalation and active sites for oxide growth. The Nb₂O₅/C composites obtained using the hydrochloric acid electrolyte exhibits a uniform coating of Nb₂O₅ on thin graphene layers (Figure 1). Finally, a mechanism for simultaneous deposition/exfoliation was proposed based on interactions between oxalatooxoniobate ions and graphite electrodes. The Nb₂O₅/C composites were tested for energy storage and exhibited good energy densities at high charge/discharge rates.